The invention concerns a method for taking into consideration control errors in the control of the ratio of a continuously variable transmission with electrohydraulic control.
A continuously variable transmission usually consists, among others, of a startup unit, a forward/reverse drive unit, an intermediate shaft, a differential, hydraulic and electronic control devices and a variator. The variator usually comprises a primary and a secondary pulley, also called primary and secondary side, the two pulleys being formed by cone pulleys disposed in pairs, and is provided with a torque-transmitting wound-around element which rotates between the two cone pulley pairs.
In such a transmission the actual ratio is defined by the running radius of the wound-around element which, in turn, is a function of the axial position of the cone pulleys.
Compared with standard mechanical transmissions, continuously variable transmissions have, in general, subject to principle, one more degree of freedom, since in addition to the selection of the reduction step to be adjusted, it is also possible here to preset and control the variation speed at which the ratio is transmitted from one operation point to the other.
In continuously variable transmissions having a wound-around element (belt, chain) as torque-transmitting part, it results from the structural design that during the change of ratio the cone pulley pairs of primary and secondary side of the variator are alternatively and complementarily to each other moved apart and drawn together by adequate control elements whereby a change is produced of the running radius of the wound-around element on the cone pulleys and thus a change of ratio between primary and secondary side.
The variator is usually hydraulically controlled. The axial displacement of the cone pulleys means here a volume change which, since the adjustment develops under power or pressure control, must be compensated by the control hydraulics, by adequate flow rate changes in the respective cone pulley pair.
The change in flow rate to be adjusted by the electrohydraulic control depends here directly on the actual variable speed of the cone pulley pairs.
Since the control hydraulic system is, as a rule, supplied via an engine-torque dependent pump with constructionally preset maximal flow rate, there necessarily results also a constructionally stationary limit for the implementable adjustment dynamics of the variator. The variator can be adjusted only as quickly as admits the available oil flow rate in the interplay with other control and regulating loops or consumers.
In the design of the supply pump, together with the assurance of the needed oil flow rates, an essential part is also played by aspects, such as noise and efficiency, both of which negatively take effect as the size of the pump increases. The result of this is that for the structural design of the pump a compromise is implemented between the different criteria which in relation to the operation point and the individual criteria is only a less than optimal solution.
In relation to the adjustable variable speeds of the variator this means that operation states will always exist in which theoretically higher variable gradients would be possible as momentarily allowed by the actual availability of the oil flow rate.
The operation states are especially critical for a superposed control device, since the control without the transmitting medium oil does not act effectively on the behavior of the variator and thus on the setting of the ratio. The consequences are instabilities which can act as interfering oscillations of rotational speed until destroying the transmission mechanics.
One other aspect is formed by the limitations on the variator subject to the design (resistances of the parts, limit values for control pressures) which, to prevent the damage or even the destruction of the transmission mechanics, likewise must at every moment be taken into consideration.
A simple possibility of implementation would be to preset for the admissible variable gradients empirical limit values which are far enough from the critical values. The disadvantage here is that the possible adjustment potential in this case cannot be utilized to the extent required. Besides, a generalization regarding the safety in all operation states is hardly possible.
Within the scope of the development, the applicant has proposed a method which by means of a physicomathematical pattern continuously calculates in each operation state the actual limit values for the maximum possible variable gradients. Here are taken into consideration the special marginal conditions of oil supply and geometric ratios on the variator subject to the design.
The superposed control device for adjusting a preset theoretical value of the ratio additionally takes into consideration the limit values when generating the correcting variables.
As basis of this variable control serves a combination of physicomathematically pattern-based linearization of the control system with a linear PID controller. The correcting variable of the PID controller is directly interpreted as standard for the variable gradients to be set.
The physicomathematical pattern used is actuated on the input side with the variable-gradient theoretical values generated by the controller as correcting variable and generates therefrom the adequate control pressures for the variator. From the point of view of control technology, what is here concerned is a purely controlled cycle based on synchronizing to a sufficient extent pattern and reality.
In practice to a sufficient extent, this is true for the main controlled variable ratio (iv) whereas in relation to the variable gradients--especially in the dynamic case--partly strong divergences related to the operation point appear. The importance of the divergences increases when the above described limit values for the variable gradients, even if adhered to by the controller when calculating the correcting variable, the subsequent (controlled) conversion to pressure values nevertheless shows that the limit values were exceeded in reality.
Accordingly, it is further needed to compensate for divergences between pattern and reality in order to implement as much as possible the objective of operation of the transmission on the "safe side" in every operation point.
Therefore, the problem on which this invention is based is departing from the cited prior art to minimize with electrohydraulic control the error in the correcting variable of variable gradient in the ratio control of a continuously variable automatic transmission.